Low-latitude sea-surface temperatures for the mid-Cretaceous and the evolution of planktic foraminifera

Isotopic data from exceptionally well preserved foraminifera show that the subtropical North Atlantic had sea surface temperatures (SSTs) between 30 and 31 degrees C during the late Albian and early Cenomanian. These temperatures were warmer than average modern temperatures in the tropical western Atlantic and support previous inferences based on diagenetically suspect material of Albian tropical SSTs of similar to 30 degrees C, This finding is consistent with basic theories of oceanic thermal response in the tropics to the high atmospheric carbon dioxide levels during the middle-Cretaceous greenhouse. However, our data do not support the concept of a super-Tethyan region of hypersaline waters in the western North Atlantic during late Albian and early Cenomanian time, because seawater salinities substantially higher than modern ones would require significantly higher SSTs than those reported here. Isotopic analysis of mid-Cretaceous planktic foraminifera shows that Planomalina, Ticinella, and Biticinella grew in surface waters and provide isotopic proxies for SST, whereas deeper growing species include Hedbergella, Rotalipora, and Praeglobotruncana. None of these taxa display isotopic indications of photo-symbiosis, suggesting that this ecology did not evolve in planktic foraminifera before the Late Cretaceous, Contrary to previous hypotheses. none of these species increased their depth habitat during their life cycles, unlike many extant planktic foraminifera. The rotaliporids may have been preferentially susceptible to extinction during oceanic anoxic events because they Lived within or below the thermocline for most of their life cycles.